## 问题 You want an easy way to execute functions or methods in Python programs running onremote machines. ## 解决方案 Perhaps the easiest way to implement a simple remote procedure call mechanism is touse XML-RPC. Here is an example of a simple server that implements a simple key-value store: from xmlrpc.server import SimpleXMLRPCServer class KeyValueServer: _rpc_methods_ = [‘get', ‘set', ‘delete', ‘exists', ‘keys']def __init__(self, address): > > self._data = {}self._serv = SimpleXMLRPCServer(address, allow_none=True)for name in self._rpc_methods_: > > self._serv.register_function(getattr(self, name)) def get(self, name):return self._data[name]def set(self, name, value):self._data[name] = valuedef delete(self, name):del self._data[name]def exists(self, name):return name in self._datadef keys(self):return list(self._data)def serve_forever(self):self._serv.serve_forever() # Exampleif __name__ == ‘__main__': > kvserv = KeyValueServer((‘', 15000))kvserv.serve_forever() Here is how you would access the server remotely from a client: >>> from xmlrpc.client import ServerProxy >>> s = ServerProxy('http://localhost:15000', allow_none=True) >>> s.set('foo', 'bar') >>> s.set('spam', [1, 2, 3]) >>> s.keys() ['spam', 'foo'] >>> s.get('foo') 'bar' >>> s.get('spam') [1, 2, 3] >>> s.delete('spam') >>> s.exists('spam') False >>> ## 讨论 XML-RPC can be an extremely easy way to set up a simple remote procedure call service.All you need to do is create a server instance, register functions with it using the register_function() method, and then launch it using the serve_forever() method. Thisrecipe packages it up into a class to put all of the code together, but there is no suchrequirement. For example, you could create a server by trying something like this: from xmlrpc.server import SimpleXMLRPCServerdef add(x,y): > return x+y serv = SimpleXMLRPCServer((‘', 15000))serv.register_function(add)serv.serve_forever() Functions exposed via XML-RPC only work with certain kinds of data such as strings,numbers, lists, and dictionaries. For everything else, some study is required. For in‐stance, if you pass an instance through XML-RPC, only its instance dictionary ishandled: >>> class Point: ... def __init__(self, x, y): ... self.x = x ... self.y = y ... >>> p = Point(2, 3) >>> s.set('foo', p) >>> s.get('foo') {'x': 2, 'y': 3} >>> Similarly, handling of binary data is a bit different than you expect: >>> s.set('foo', b'Hello World') >>> s.get('foo') <xmlrpc.client.Binary object at 0x10131d410> >>> _.data b'Hello World' >>> As a general rule, you probably shouldn’t expose an XML-RPC service to the rest of theworld as a public API. It often works best on internal networks where you might wantto write simple distributed programs involving a few different machines.A downside to XML-RPC is its performance. The SimpleXMLRPCServer implementa‐tion is only single threaded, and wouldn’t be appropriate for scaling a large application,although it can be made to run multithreaded, as shown in Recipe 11.2. Also, sinceXML-RPC serializes all data as XML, it’s inherently slower than other approaches.However, one benefit of this encoding is that it’s understood by a variety of other pro‐gramming languages. By using it, clients written in languages other than Python will beable to access your service.Despite its limitations, XML-RPC is worth knowing about if you ever have the need tomake a quick and dirty remote procedure call system. Oftentimes, the simple solutionis good enough.